GE Corometrics 170 Series Service Manual page 158

7-26
Serviceable Assemblies: Tocotransducer Calibration
NOTE: When performing this procedure, keep in mind that due to the nature of the
circuitry in the Trimline Tocotransducer, there is a stabilization time of
approximately two seconds for any output voltage. After adjusting a
potentiometer, verify that the output has stabilized prior to taking a voltage
reading.
1. Remove the nameplate and sealing tape from the top cover of the Trimline
Tocotransducer.
2. Set the GAIN and ZERO CTR potentiometers in the tocotransducer fully
counterclockwise.
3. Set the ZERO TRIM potentiometer to its mid-position—approximately six
turns from fully counterclockwise.
4. Locate TP2 and TP9 (ground) on the Main Board in the 170 Series Monitor.
5. Connect the positive lead of the digital voltmeter to TP2; connect the negative
lead to TP9.
6. Apply power to the 170 Series Monitor; then plug the CMR Test Jack into the
monitor's front panel uterine activity connector.
7. Measure the voltage at TP2 and record the value.
8. Replace the CMR Test Jack with the tocotransducer to be calibrated.
9. Place the tocotransducer on a flat surface so that the button is face up, then
center the 52.5 gram weight on the button.
10. Adjust the ZERO CTR and/or the ZERO TRIM potentiometer(s) so that the
voltage measured across TP2 and TP9 matches the reading recorded in step 7, ±
0.04 V. This voltage is referred to as the WEIGHT ON voltage.
The ZERO CTR potentiometer provides course adjustment.
The ZERO TRIM potentiometer provides fine adjustment.
11. Remove the weight and observe the voltage reading; this voltage is referred to
as the WEIGHT OFF voltage. A correctly adjusted transducer will show a
voltage change of +0.115 V ± 0.015 V from the WEIGHT ON to WEIGHT
OFF voltage. If the change is less than 0.1 V, adjust the GAIN potentiometer
counterclockwise to increase the gain. If the change is more than 0.13 V, then
adjust the GAIN potentiometer clockwise to decrease the gain.
12. Repeat step 9 through step 11 until the change in voltage between the WEIGHT
ON and WEIGHT OFF conditions falls into specification.
170 Series Monitor
2000947-004
Revision C